The present invention relates to devices and methods for stabilizing the spine. It more particularly relates to anterior stabilization of a spinal column (backbone) that has been rendered unstable by any disease such as a tumor, infection, or congenital anomalies affecting the vertebral bodies or associated supporting structures.
Preservation of neurologic function and relief of pain are the primary goals of treatment of diseases affecting the vertebral bodies. Oftentimes such diseases themselves destroy the vertebral body or the surgical treatment necessitates its resection. As a result of the loss of bony support, the vertebral column is unstable causing pain, progressive neurologic deterioration, and physical deformity of the back. In such cases stabilization of the spinal column by an anterior surgical approach may be required.
Each of the various procedures described in the past, however, has its individual shortcomings. No entirely satisfactory method until the present invention has been developed for effectively stabilizing the spine from the anterior surgical approach.
The most commonly used method of anterior spine stabilization is to fill the space of the resected vertebral bodies with methylmethacrylate or other similar plastic or plastic-like polymers. Difficulties encountered with this method include: (1) the inability to bond securely the plastic material to tissue or bone, (2) the potentially injurious heat released into the surrounding tissue because of the exothermic chemical reaction required for polymerization of the plastic, (3) potential stress fractures of the plastic caused by mechanical forces related to weight bearing, and (4) the need to mold the plastic to the appropriate size which is often technically cumbersome and difficult. Also, to secure the methylmethacrylate adequately, metallic pins or struts must be incorporated into the molded methylmethacrylate after their impaction or penetration into the adjacent vertebral bodies (vide infra).
Another common method of anterior spine stabilization utilizes bone to act as a replacement for the vertebral body after its removal because of disease. Bone of the iliac crest, fibula, or tissue-banked cadaver bone has been used. However, several limitations of this technique exist: (1) Immediate stabilization is not possible because adequate bony fusion requires three to six months. As a result, such individuals must wear an external spinal stabilization device for weeks or months postoperatively. Patient tolerance of such externally worn devices is often poor. (2) Oftentimes, especially when radiation therapy and/or chemotherapy are required to treat the primary disease process affecting the spine, the implanted bone graft does not survive and this results in further spinal instability secondary to resorption of the bone graft. This shortcoming has attempted to be overcome by a surgical procedure in which a bone graft on a vascularized pedicle is used since it is felt that most failures are due to inadequate blood supply to the graft. However, this procedure is technically difficult and therefore not commonly used. Even under the best of circumstances, the bone graft is partially resorbed and collapse of the vertebral space can lead to compression of the neural tissues in the spinal canal.
Attempts at spinal stabilization have also been performed utilizing a variety of metal pins and struts as mentioned previously. One common method is to penetrate adjacent vertebral bodies with Steinmann pins and then surround the pins with methylmethacrylate. Many technical problems are associated with this procedure including (1) those difficulties specially related to methylmethacrylate; (2) the inability to maintain distraction adequately while polymerization of the plastic is taking place; (3) the inability to adequately place the pins because of space restrictions; and (4) the fracture of the pins resulting in loss of stability and possible movements of the methylmethacrylate.
Another method for anterior spinal stabilization utilizes one or two metal rods attached by screws to the lateral aspect of the adjacent vertebral bodies. This procedure was first described by Dunn in 1980. There are two primary limitations of this procedure. First, it cannot be used in certain areas of the spine because of the vital structures adjacent to the lateral aspect of the vertebral body. Second, the device is inherently weak since mechanical stresses are maximally exerted at the site of attachment of the device to the vertebral body by small metal screws.
Metal rod distraction utilizing Harrington or Knodt rods, with or without methylmethacrylate, is another method to stabilize the spine via the anterior approach. This procedure was described by Harrington in 1976. One or more rods are typically attached via hooks to the anterior portion of the vertebral body and they can then be covered with methylmethacrylate to further secure them in place. The major disadvantages of this technique is the slippage of the stabilizing rods secondary to the mechanical forces generated by spine movement. These forces are greatest at the site of hook insertion and may cause fracture of the vertebral body to which it is attached. Moreover, involvement of the adjacent vertebral bodies with disease may weaken the bone at the site of hook insertion and lead to migration of the rods with resultant collapse of the spinal column.
Accordingly, the principal object of the present invention is to provide a novel device and method for stabilizing the spinal column but without the shortcomings and disadvantages of the prior methods as set forth above.
Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the foregoing description taken in conjunction with the accompanying drawings.